Exemplary embodiments herein relate to an intelligent stepper welding system and method for controlling weld current in a welding operation.
Spot welding is a commonly employed resistance welding technique used to join metallic work pieces. Spot welding operates by passing electric current through the work pieces to be joined. This causes a localized heating of the work pieces that is sufficient to produce a molten weld pool therebetween. Upon cooling, the weld pool forms a weld nugget that joins the work pieces. Spot welding, such as by automated robots, is often used to manufacture a body-in-white assembly on a vehicle assembly line.
As it is desirable to conduct the electric current as efficiently as possible, conducting electrodes are employed to contact the work pieces at the welding location. Although various conductive metals may be used, copper is commonly utilized in the manufacture of weld electrodes. Spot welding additionally requires that the weld electrodes be forcibly pressed against the work pieces during the welding process. As is known, the electrodes are prone to wear and/or damage during use (i.e., mushroom), particularly electrodes made of copper or another similar softer conductive material. For example, weld electrodes wear down and/or become deformed during use as the result of heat generated by the welding process, and also due the force the electrodes are required to exert the work pieces upon which they operate.
To keep the welding electrodes in satisfactory welding condition, the welding electrodes are typically subjected to a periodic cutting or shaping process commonly referred to as “tip dressing.” This limits the overall amount of deformation or mushrooming of a weld electrode tip, but significant deformation can still occur between tip dressings. In a vehicle manufacturing line, one example tip dressing schedule requires the electrodes to be dressed approximately every twenty vehicles or vehicle bodies. Thus, if there were twenty welds per vehicle body that a welding electrode must perform, there could be upwards of 400 individual spot welds that are performed by the welding electrode between tip dresses (twenty vehicle×twenty spot welds). By way of example, a 6 millimeter tip face could deform or mushroom to approximately 8 millimeters in diameter, which would correspond to the facial area of the electrode tip increasing from about 28 mm2 to around 50 mm2.
One solution for dealing with deforming electrode tips between tip dresses is to crank up the initial welding current high enough such that individual spot welds are still satisfactory when the electrode tips deform, such as to 8 millimeters in diameter. Unfortunately, a side effect of such increased welding current is that weld spatter increases greatly, as does porosity of the welds. Another alternative is to increase the tip dress frequency for the electrodes, such as to approximately every three vehicles. However, this results in slower production of vehicle bodies and thus increased production costs.